IAM policy grants AssumeRole permission across all services

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Metadata

Id: cloudformation-aws-iam-policy-grants-assumerole-permission-across-all-services

Provider: AWS

Platform: CloudFormation

Severity: Medium

Category: Access Control

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Description

IAM policies must not grant the sts:AssumeRole action against all resources (*), because allowing AssumeRole on * enables principals to assume any role and can lead to privilege escalation and broad lateral movement. Check AWS::IAM::Policy resources’ Properties.PolicyDocument.Statement entries for Effect: Allow with Action containing sts:AssumeRole (case-insensitive) and Resource equal to * or containing *. Statements that allow sts:AssumeRole must instead restrict Resource to explicit role ARNs or a limited set of ARNs (for example, arn:aws:iam::123456789012:role/MyRole). Resources missing this restriction or with Resource: "*" will be flagged.

Secure configuration example (restrict the resource to a specific role ARN):

MyPolicy:
  Type: AWS::IAM::Policy
  Properties:
    PolicyName: RestrictAssumeRole
    PolicyDocument:
      Version: "2012-10-17"
      Statement:
        - Effect: Allow
          Action: sts:AssumeRole
          Resource: arn:aws:iam::123456789012:role/SpecificRole
    Roles:
      - Ref: SomeRole

Compliant Code Examples

AWSTemplateFormatVersion: "2010-09-09"
Description: A sample template
Resources:
  MyPolicy:
    Type: AWS::IAM::Policy
    Properties:
      PolicyName: mygrouppolicy
      PolicyDocument:
        Version: "2012-10-17"
        Statement:
          - Effect: Allow
            Action:
              - s3:GetObject
              - s3:PutObject
              - s3:PutObjectAcl
            Resource: arn:aws:s3:::myAWSBucket/*
      Groups:
        - myexistinggroup1
        - !Ref mygroup

Non-Compliant Code Examples

AWSTemplateFormatVersion: "2010-09-09"
Description: A sample template
Resources:
  mypolicy:
    Type: AWS::IAM::Policy
    Properties:
      PolicyName: mygrouppolicy
      PolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Action: ["sts:AssumeRole"]
          Resource: "*"
      Users: ["SomeUser"]